1. Field of the Invention
This invention relates generally to quartz crystal microbalance systems, and to a process and apparatus for delivery of a product gas stream containing a desired concentration level of a specific gas component.
2. Description of the Related Art
In many applications and industrial processes, there is need to deliver a multicomponent gas stream as a product containing a particular concentration of a specific gas component.
One such field is semiconductor manufacturing, in which the product gas stream may include one or more reagents, either with or without a carrier gas, for vapor deposition processes in a deposition chamber. The deposition process may be carried out to form a layer or structure in which the thickness and composition of the deposited material is critical to the operability and function of the resulting product device. Particularly when multicomponent deposition operations are being conducted, and stoichiometry of the deposited film is critical, the concentration of specific components in the product gas stream requires control within strict tolerances to meet process objectives.
A specific example in semiconductor manufacturing is trichlorosilane delivery from bubbler systems for formation of silicon and/or silicon oxides. In conventional trichlorosilane bubbler delivery systems, the variation in film thickness of the silicon-containing material is on the order of 5%-10% across the surface of the film. Such thickness variation is unacceptable for many applications.
Specifically, this level of thickness variability may necessitate the rejection of significant amounts of product articles, due to cumulation of the thickness variation through the succeeding material layers deposited on the silicon-containing material. Additionally, or alternatively, such variation in thickness may require remedial treatment to planarize or otherwise compensate for the variation in the applied film thickness.
The measure of the effectiveness of the control on the deposition process includes a number of measurement parameters. Illustrative parameters include accuracy, precision and resolution. Accuracy is a measure of how close a measured value is to the actual value, and is an important factor in the measurements used in process control. Precision is a measure of the reproducibility of the measurement. The precision measurement may have span or offset errors but the measurement and the process can still remain reproducible. Resolution is the ability to divide the measurement into fine segments of the whole.
These illustrative parameters can be expressed as bit values. A bit value is 2 to the bit power. For example, 8-bit is 2.sup.8 or 256. A semiconductor manufacturing process capable of achieving 9-bit precision would significantly improve the control and resulting quality of the deposition process and the product.
For example, in order to achieve 9-bit precision or accuracy, an instrument must be capable of resolving one part in 512, or 0.2%. Resolution of one part in 512 achieves 9-bit precision, if the results are reproducible. With no span or offset errors, 9-bit resolution and 9-bit precision results in 9-bit accuracy.
Drift or span or offset will degrade accuracy but will not necessarily degrade precision. Further, if the drift is slow on the time scale of a process being controlled, such as a wafer run, then the loss of accuracy may not be a significant impediment to control of the deposition process.
If the concentration or partial pressure of a process chemical such as TCS could be accurately, precisely and reproducibly measured, then a feedback loop control system could be utilized to maintain the flux of the process chemical into the tool at a higher level or accuracy and precision than heretofore possible.
Quartz microbalance systems are known in the art for monitoring gas concentrations.
One such system for monitoring components of a fluid or gaseous mixture is disclosed in U.S. patent application Ser. No. 08/785,342 filed Jan. 17, 1997 (now U.S. Pat. No. 5,827,947) in the names of Cynthia A. Miller and Glenn M. Tom for "Piezoelectric Sensor for Hydride Gases, and Fluid Monitoring Apparatus Comprising Same," the specification of which is hereby incorporated herein by reference in its entirely.
Other quartz microbalance systems are described in U.S. Pat. No. 4,637,987 to Minten et al. and U.S. Pat. No. 5,065,140 to G. G. Neuberger.
In quartz microbalance gas detection systems, a quartz crystal element with one or more specialized coatings on the crystal may be utilized to monitor gas concentration. The crystal is oscillated by an electric field at a frequency determined by the mass of the crystal. Any change in crystal mass will result in a change in the oscillation frequency. As the coating adsorbs or reacts with specific gas constituents, the mass of the crystal will change.
The oscillation frequency of the quartz crystal may therefore be monitored to determine gas concentration, taking advantage of the fact that when the mass of the crystal increases, the oscillation frequency will decrease, thereby indicating an increase in the concentration of a specific gas constituent.
Accordingly, it would be a significant advance in the art, and accordingly is an object of the present invention, to provide a gas stream supply system in which TCS or other component is supplied to a semiconductor manufacturing deposition process at a precise concentration in the delivered gas stream, so that the resulting deposited thickness of the material deposited from such stream is highly uniform in character.
It is another object of the invention to provide a quartz microbalance sensor of high resolution characteristics for detection and measurement of gas concentration in such applications.
It is yet another object of the present invention to provide a control system for delivery of a gas component of a multicomponent gas stream at a selected concentration with a low level of variance in the gas concentration.
It is another object of the present invention to provide a highly efficient detector for determining the concentration of a selected component in a multicomponent gas stream.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.